Compared to other synthetic resins, cellulose resins are mainly characterized by generally having a superior strength, transparency and gloss, as well as a smoother surface with excellent texture. Because of this, cellulose resins are used in a wide variety of applications, for example, sheets, films, wire coatings, toys, medical instruments, food packaging materials and the like.
However, since cellulose resins are not thermoplastic, there has been problems in that these resins, when molding, have to be melted at a high temperature or dissolved into a solvent, and that in cases where they are melted at a high temperature, they are simultaneously thermally decomposed and become colored. Therefore, in order to avoid such problems, it is necessary to blend an appropriate plasticizer which lowers the softening point of the cellulose resin. Examples of conventionally used plasticizer include triphenyl phosphate, tricresyl phosphate, diphenyl phosphate, triethyl phosphate, tributyl phosphate, dimethyl phthalate, diethyl phthalate, dimethoxyethyl phthalate, ethyl phthalyl ethyl glycolate, butyl phthalyl butyl glycolate, toluenesulfonamide, triacetin and pentaerythritol tetraacetate.
However, at present, none of the aforementioned plasticizers satisfies a wide range of performances such as compatibility with cellulose resins, plasticizing efficiency, non-volatility, stability against heat and light, non-migratory property, non-extractability and water resistance, and this is discouraging a further expansion of the use of cellulose resin compositions.
In view of this, a plasticizer having a hydrocarbon ring was proposed in Patent Document 1 and an ester compound of polyalcohol having different alkyl groups and aryl groups was proposed in Patent Document 2; however, these were not satisfactory.
In addition, recent years have seen the development in thinning and weight reduction of information devices equipped with a liquid crystal display such as laptop computers. At the same time, the demands for even thinner protective film for polarizing plate used in liquid crystal displays have increased.
Meanwhile, conventionally, cellulose triacetate films have been commonly and widely used for a protective film for polarizing plate, and a plasticizer has been added to such cellulose triacetate films in order to improve the film flexibility and to reduce the moisture permeability.
However, there have been cases where simple thinning of the aforementioned cellulose triacetate film increased the moisture permeability of the film and made it impossible to sufficiently reduce the moisture, thereby deteriorating the adhesive and polarizer when the film was made into a polarizing plate. As the solution to this problem, it can be considered to increase the amount of the plasticizer to compensate the amount reduced by the thinning; however, a mere increase in the amount of the plasticizer does not enable a sufficient improvement in the moisture permeability, and it has been found that such an increase in the amount of the plasticizer causes new problems, such as deterioration in the retainability. Here, deterioration in the retainability refers to a decrease of the film mass due to precipitation, evaporation and the like of the additive such as plasticizer to the outside of the film under a high temperature and humid environment. This retainability is poor in conventional cellulose ester films, resulting in a decrease in the performance of liquid crystal displays.
In view of the above, as the technique to improve these defects, Patent Document 3 proposes the use of citrate, while the use of pentaerythritol ester and dipentaerythritol is proposed in Patent Document 4. In addition, Patent Document 5 proposes the use of glyceride, while the use of diglycerol ester is proposed in Patent Document 6.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 53-40039
Patent Document 2: Japanese Unexamined Patent Application Publication No. 54-71141
Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-92574
Patent Document 4: Japanese Unexamined Patent Application Publication No. 11-124445
Patent Document 5: Japanese Unexamined Patent Application Publication No. 11-246704
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2000-63560